In the operation of steel mill and similar production facilities large tonnages of rejected screenings and waste are produced in the form of ore fines, furnace dusts, water treatment plant sludges, grindings, borings, scarfings, coke fines, mill scale, slags, and other materials containing valuable constituents, such as iron oxide and combustible elements, which range in size from sub-micron to as large as three-eighths inch but which are too finely divided to feed into a melting furnace. The feeding of such fine materials causes them to be blown out of the combustion zone before melting or chemical conversion can take place.
Thus it has been recognized for many years that if such screenings and wastes are to be utilized to recover their constituents including any fuel value which they contain, it is necessary to form the materials into agglomerated form.
In the past agglomeration of ores, concentrates, screenings and wastes has been accomplished by one or more of three procedures: pelletizing, sintering and briquetting.
Both pelletizing and sintering require a high plant investment and have high operating costs. Because of the large scale on which such processes must be carried out and the large quantities of air and other gases which must be handled, pelletizing and sintering are not well suited to utilization of the more limited quantities of waste and rejected screenings available at many plant locations. The pelletizing and sintering plants now in use require large amounts of fuel, sintering plants being particularly inefficient in the use of fuel. Also, both processes require complete combustion of non-volatile combustible matter in the material being processed. Objectionable volatile matter, such as the oil frequently present in mill scale and water plant sludges, can present serious pollution problems due to its presence in the gases evolved from pelletizing and sintering plants.
With respect to briquetting, the process has received only limited acceptance. Agglomeration at low temperature by compaction without binders produces a fragile briquette and the use of binders is too expensive for most applications. Cold-bonded briquettes usually are not acceptable as a furnace feed material because they tend to disintegrate within the furnace prior to melting. Hot briquetting, on the other hand, has not reached a mature state of development or gained acceptance because large quantities of ore fines have been generally available at the steel mills for mixing with the wastes, so that large sintering plants using well-known processing techniques could be built and operated economically. With the advent of pelletizing plants located at the mine sites to process the ore fines and concentrates, and with the increased cost of energy and pollution controls for sintering plants, it is no longer economical to construct sintering plants at most mill sites, and many existing sintering plants have been shut down for these reasons.
It is, accordingly, an object of the present invention to provide a plant, or system, capable of accepting a wide variety of feed materials, such as rejected screenings, mill wastes, ore fines, ore concentrates and other materials having heat-softenable constituents, and capable of reliably forming such materials into highly durable briquettes, by hot briquettes which will hold together during the handling, feeding and smelting process as, for example, in a blast furnace, for melting and chemical conversion and without reverting to fines or flue dust.
It is another object of the present invention to provide a hot briquetting plant for such feed materials which is highly economical, requiring a relatively low initial investment as compared to other plants intended for the same purpose and with low maintenance and operating costs.
It is a more specific object of the present invention to provide a hot briquetting plant for steel mill screenings and waste which utilizes, efficiently, as a source of heat, the carbon and other combustible elements which form a proportion of the waste materials, by novel use of heat exchangers. In this connection it is an object to provide a hot briquetting plant which requires only a minimum of expensive auxiliary fuel such as gas, oil or coal, utilizing such auxiliary fuel only during start-up and as might be required from time to time when processing waste having an unusually low proportion of combustion supporting materials. In this connection, also, it is an object to provide a briquetting plant in which materials are largely fed by gravity, due to the novel construction of the plant, and which requires only a relatively small amount of conveying equipment and a minimum of mechanical energy.
It is still another object of the present invention to provide a hot briquetting plant which is readily automated and which is capable of operating stably and continuously with a minimum of supervision and man power.
It is a further object of the invention to provide a hot briquetting plant which is not dependent upon large scale operation and in which the component equipment may be scaled down in size and capacity without a corresponding reduction in efficiency, making the design of plant suitable for use on a continuous basis to process materials for small, as well as large, industrial operations.
It is yet another object of the invention to provide a hot briquetting process which inherently produces a minimum of effluent gases and in which the quantity and type of gases are such as to be processed using low cost anti-pollution equipment as necessary to meet local pollution standards.
It is a related object to provide a plant which is compact and which utilizes variable output feeders and conveyors for controlling flows to and throughout the plant so that the storage capacity required within the system is small and can be included within the heat exchangers and the furnace, thereby avoiding any need for separate in-plant storage vessels.